The Dispatcher servlet

We put values into the model, and we created the view that can read those values from the model. So, the Controller acts as an intermediate between the View and the Model; with this, we have finished all the coding part required to present the welcome page. So will we be able to run our project now? No. At this stage, if we run our project and enter the URL http://localhost:8080/webstore/ on the browser, we will get an HTTP Status 404 error. This is because we have not performed any servlet mapping yet.

Tip

So what is servlet mapping? Servlet mapping is a configuration of mapping a servlet to a URL or URL pattern. For example, if we map a pattern like /status/* to a servlet, all the HTTP request URLs starting with a text status such as http://example.com/status/synopsis or http://example.com/status/complete?date=today will be mapped to that particular servlet. In other words, all the HTTP requests that carry the URL pattern /status/* will be handed over to the corresponding mapped servlet class.

In a Spring MVC project, we must configure a servlet mapping to direct all the HTTP requests to a single front servlet. The front servlet mapping is a design pattern where all requests for a particular web application are directed to the same servlet. This pattern is sometimes called as Front Controller Pattern. By adapting the Front Controller design, we make front servlet have total control over the incoming HTTP request so that it can dispatch the HTTP request to the desired controller.

One such front servlet given by Spring MVC framework is the Dispatcher servlet (org.springframework.web.servlet.DispatcherServlet). We have not configured a Dispatcher servlet for our project yet; this is why we get the HTTP Status 404 error if we run our project now.

Time for action - configuring the Dispatcher servlet

The Dispatcher servlet is what examines the incoming HTTP request and invokes the right corresponding controller method. In our case, the welcome method from the HomeController class needs to be invoked if we make an HTTP request by entering the http://localhost:8080/webstore/ URL on the browser. So let's configure the Dispatcher servlet for our project:

1. Create a class called WebApplicationContextConfig under the com.packt.webstore.config package in the source directory src/main/java and add the following code into it:

         package com.packt.webstore.config; 
    
         import org.springframework.context.annotation.Bean; 
         import org.springframework.context.annotation.ComponentScan; 
         import org.springframework.context.annotation.Configuration; 
         import 
         org.springframework.web.servlet.config.annotation
         .DefaultServletHandlerConfigurer; 
         import org.springframework.web.servlet.config.annotation
         .EnableWebMvc; 
         import org.springframework.web.servlet.config.annotation
         .WebMvcConfigurerAdapter; 
         import org.springframework.web.servlet.view
         .InternalResourceViewResolver; 
         import org.springframework.web.servlet.view.JstlView; 
    
         @Configuration 
         @EnableWebMvc 
         @ComponentScan("com.packt.webstore") 
         public class WebApplicationContextConfig extends 
         WebMvcConfigurerAdapter { 
    
              @Override 
             public void configureDefaultServletHandling
             (DefaultServletHandlerConfigurer configurer) { 
                 configurer.enable(); 
                                  } 
     
             @Bean 
             public InternalResourceViewResolver 
             getInternalResourceViewResolver() { 
                 InternalResourceViewResolver resolver = new 
                 InternalResourceViewResolver(); 
                 resolver.setViewClass(JstlView.class); 
                 resolver.setPrefix("/WEB-INF/jsp/"); 
                 resolver.setSuffix(".jsp"); 
    
                 return resolver; 
             } 
         } 
   

2. Create a class called DispatcherServletInitializer under the com.packt.webstore.config package in the source directory src/main/java and add the following code into it:

         package com.packt.webstore.config; 
    
         import org.springframework.web.servlet.support
         .AbstractAnnotationConfigDispatcherServletInitializer; 
    
         public class DispatcherServletInitializer extends 
         AbstractAnnotationConfigDispatcherServletInitializer { 
    
              @Override 
              protected Class<?>[] getRootConfigClasses() { 
                   return null; 
              } 
    
              @Override 
              protected Class<?>[] getServletConfigClasses() { 
                   return new Class[] { 
                   WebApplicationContextConfig.class }; 
              } 
    
              @Override 
              protected String[] getServletMappings() { 
                   return new String[] { "/" }; 
              } 
         } 
   

What just happened?

If you know about servlet programming, you might be quite familiar with the servlet configuration and web.xml, and in that case, you can consider the DispatcherServletInitializer class as similar to web.xml. In step 2, we configured a servlet of type DispatcherServlet by extending the AbstractAnnotationConfigDispatcherServletInitializer class, which is more or less similar to any other normal servlet configuration. The only difference is that we have not instantiated the DispatcherServlet class for that configuration. Instead, the servlet class (org.springframework.web.servlet.DispatcherServlet) is provided by the Spring MVC framework and we initialized it using the AbstractAnnotationConfigDispatcherServletInitializer class.

After this step, our configured DispatcherServlet will be ready to handle any requests that come to our application on runtime and will dispatch the request to the correct controller's method.

However, DispatcherServlet should know how to access the controller instances and view files that are located in our project, and only then can it properly dispatch the request to the correct controllers. So we have to give some hint to DispatcherServlet to locate the controller instances and view files.

This is what we configured within the getServletConfigClasses method of the DispatcherServletInitializer class. By overriding the getServletConfigClasses method, we are telling DispatcherServlet about our controller classes and view files. And in step 1, through the WebApplicationContextConfig class file, we configured the InternalResourceViewResolver and other default configurations.

Don't worry if you are not able to understand each and every configuration in the WebApplicationContextConfig and DispatcherServletInitializer classes; we will take a look deep into these configuration files in next chapter. As of now, just remember that this is a one-time configuration that is needed to run our project successfully.